This invention relates to a process for preparing solid rocket propellants. More particularly, this invention relates to a low Shear Mixing process particularly adapted for mixing high solids, high viscosity, solid rocket propellants.
The present high interest in the operation of rockets and missiles has spawned a considerable research effort in the development of procedures and techniques for preparing and mixing solid rocket propellants. A number of problems have been encountered however, during the preparation of high solids, high viscosity propellant systems because of their sensitivity to friction and high temperature. Heretofore, the most common technique for the manufacture of solid rocket propellants involved the use of blade-type mechanical mixers. A variety of such mixers are used in the propellant manufacturing industry, including horizontal sigma blade, Ko-Kneaders, vertical single and double planetary, conical, and helical designs. These types of mixers are designed for heavy duty work, and the component tolerances are accurately controlled.
These mechanical high shear mixer systems, however, are limited as high solids, high viscosity propellant systems are developed. These propellant systems are, or may be characterized by high sensitivity relative to friction and temperature, short pot life, particle degradation, high transition potential, and high solids loading and viscosity. Relative to these new propellant systems, the mechanical mixers have inherent limitations. For example, the mixing of a high solids, high viscosity propellant involves considerable energy input resulting from the close tolerance, high shear mixing action. The Mixers impart high stresses to the driving and mixing components when processing these high viscosity formulations. This not only constitutes a hazardous condition but adversely effects and degrades propellant properties. Also, the elevated temperature mixing coupled with mix/cast cycle time frequently limits the processing of short pot life propellants. In addition, the high shear mixing action and close blade tolerances may be detrimental to ingredient particles or particle coating integrity which results in increased sensitivity problems.
Circumvention of the high shear mixing constraints and the pot life problem of propellant manufacture encountered with prior known mixing techniques have been overcome by the low shear processing concept of this invention. The concept includes operations from propellant mixing through casting and will be discussed in greater detail hereinafter. The principal process steps, however, are comprised of diluent mixing, propellant extrusion/freezing operations, diluent removal by freeze-drying and casting, all of which impart minimal shear force characteristics to a propellant mix.